How do you know things would return to normal?

You’ve hit the nail on the head by introducing the possibility a continued momentum caused by positive feedbacks. The simple fact is that we don’t know for sure what would happen if human beings disappeared.

We can model different scenarios but the variability within the outputs would be huge, partly because of all the uncertainties and also because of the time scales involved. If we’re talking about 1,000 years then a tiny error at the start of the run could multiply into a very significant error at the end of the run.

Everything in this scenario hinges on multiple tipping points. People often refer to a climate tipping point as being a singular entity, but there are hundreds, thousands, millions of them. Each individual species (of which there are millions) will have it’s own tipping point.

The problem is that we can’t predict in advance with any degree of certainty where these tipping points lie. In the years to come we’ll be able to look back and identify when they happened, as we already can with some of the tipping points that we’ve already crossed; but this is something that can only accurately be identified with hindsight.

That said, the degree of heating or cooling as a consequence of natural cycles is of a magnitude many times greater than the climatic variation needed to cross a tipping point threshold. The only exceptions are the most pronounced of positive feedbacks such as the decline of sea-ice extent.

This is the factor that is most likely responsible for the level of warming observed in the Arctic, the part of the planet that has warmed significantly more than anywhere else. We may already have crossed the threshold such that the coupled reaction between loss of albedo and enhanced warming has become unstoppable.

What would happen if the human species did disappear is that greenhouse gas emissions would immediately return to their natural and balanced levels. Those greenhouse gases that we’ve already emitted would reside in the atmosphere for decades to come, the overall average being 84 years, but some of the synthetic gases (the CFC’s, HFC’s, HCFC’s etc) would remain indefinitely; although it should be noted that their contribution to global warming is very small.

If we disappear completely, and if we haven’t crossed any critical tipping points, and if the positive feedback cycles are broken following our departure, then the world would return to ‘normal’ after about a century. The average global temperature would return to levels last seen around 1980.

However, not everything would return to normal. For example, many plant and animals species have expanded their territories by generally tracking the movement of isotherms, if the world were to cool back to pre GW levels then many of these species would remain in their expanded territories where they have now become established (sometimes to the detriment of native species which would not be able to re-establish themselves if the world cooled).

What the original poster said is, in essence, correct – provided always that feedback mechanisms are broken and the changes thus far are reversible, and that’s something we really don’t know.

What is normal? Your whole premise is wrong.

Quote by Will Happer, Princeton University physicist, former Director of Energy Research at the Department of Energy: “I had the privilege of being fired by Al Gore, since I refused to go along with his alarmism....I have spent a long research career studying physics that is closely related to the greenhouse effect....Fears about man-made global warming are unwarranted and are not based on good science. The earth's climate is changing now, as it always has. There is no evidence that the changes differ in any qualitative way from those of the past.”

<Again, the problem here is that so many posters are clueless that certain questions can only be determined by mathematics.>

The Earth has been cooling for over a decade, yet CO2 level has increased. How does that math work out for you?

Quote by Chris Folland of UK Meteorological Office: “The data don't matter. We're not basing our recommendations [for reductions in carbon dioxide emissions] upon the data. We're basing them upon the climate models.”

How does that math work out for you?

In the US we have Congress which cannot balance a budget, or worse, even come up with a budget. And you want these 'mathematicians' to enact laws to provide a solution to an non-existing problem?

You premise is wrong, your solution is wrong and your conclusion is wrong.

CO2 in earths history was probably the same as Mars and Venus 90odd% and then cyanobacteria and other life formed and gave us oxygen, but since that time life has been sequestering CO2 locking it away in fossil fuels,limestone chalk and other carbonates, CO2 has been spiraling downward over billions of years down toward a less fertile planet (mainly grasses and Savannah's C4 plants) until man came to the rescue.

Any thing is possible. But likely is a different question.

Could the carbon dioxide we have added to the atmosphere remain forever? Maybe. But we should not be making claims that our actions could effect what happens tens or hundreds of thousands of years from now. And we would have to live that long to confirm that computer models which would predict that the carbon dioxide would remain in the atmosphere are accurate. At best, computer models can only work with what we actually know. They are not the Psychic Hot Line.

i think its this way

The sun spins

And currently its at the toppest temp and stuff

And so the co2 increase and decrease depending on the suns position

Also i think the co2 levels were 92 or something and global greening happened

This was about 1millionish years ago

What exactly is "normal"??

I have yet to hear what the optimum climate for Earth is.

I suspect that if this catastrophic AGW thing becomes established as a Golden Goose for tax revenues, "normal" will become a moving target. As we all know, once government gets their hands on taxpayer dollars, they will not let go.....no matter how "normal" our climate becomes.